This invention pertains generally to phase-coded pulse compression radars, and more particularly to an improved pulse compression technique for such type of radars to reduce range sidelobes.
As is known in the art, a long coded pulse is transmitted from a pulse compression radar and echo signals are processed to form relatively narrow received pulses. An increased detection capability is thereby achieved along with a range resolution capability approaching that of a narrow pulse radar system. In general, the long coded pulse is achieved by using either a linear frequency-modulated (FM), or chirp, waveform or a phase-coded waveform to modulate a carrier signal. A phase-coded waveform differs from an FM waveform in that the long transmitted pulse is subdivided into a number of shorter subpulses of equal time duration, with each such subpulse having a discrete phase that is changed at the Nyquist rate. In the radar receiver echo signals are processed utilizing a correlation technique to produce compressed pulses.
Of the many known phase-coded waveforms, the so-called "pseudo-random" code or "maximal-length sequence" are for use as the long coded pulse in a pulse compression radar because of the ease with which such codes may be generated and the desirability of the corresponding autocorrelation functions. Thus, a pseudo-random code may be generated in a shift register employing linear feedback whereby the contents of selected stages of such register are summed to form a coded input signal. With properly chosen feedback connections the output signal from the shift register is a sequence of maximal length. The length "N" of a maximal length sequence is equal to the number of subpulses in the sequence as well as the time-bandwidth product of the radar system.
The autocorrelation function (i.e., the ambiguity function along the range axis) of a pseudo-random code is either periodic (when the code generator is operated continuously), or aperiodic (when the code generator is operated for only a single complete sequence). When the autocorrelation function is periodic, the period may be expressed as "Nt", where N is the number of subpulses in the sequence and t is the time duration of each subpulse. The sidelobe level for such a function is constant (of unity magnitude). On the other hand, the autocorrelation function for an aperiodic sequence is characterized by a single peak of amplitude N at the origin (t=o) and a sidelobe structure having odd symmetry along the range axis.
Pseudo-random sequences may be generated by either a simple binary code wherein the phases of the subpulses alternate between 0.degree. and 180.degree. or by means of a polyphase code as, for example, a Frank polyphase code. In general, phase-coded waveforms are not practical in applications where high Doppler frequencies are expected because one cycle of Doppler shift during a sequence results in complete decorrelation. Even when smaller Doppler shifts may be experienced, the range resolution possible with a signal having a phase code is limited by increased sidelobe levels. Any degradation in range resolution has, of course, an adverse effect on the capability of a radar, particularly a tracking radar in a fixed emplacement. In such a radar echo signals received in range sidelobes may easily obscure desired echo signals received in the main lobe. As is known, weighting techniques may be employed to reduce range sidelobes associated with pulse compression. However, weighting reduces sidelobes by moving the sidelobe energy to widen the main lobe, thereby increasing main lobe clutter and reducing resolution of the main lobe.